Cells are surrounded by plasma membranes. Plasma membranes contain components called glycosphingolipids inserted therein which aide in the formation of the characteristic surface structure of the cells. Each type of cell is characterized by a specific profile of the glycosphingolipid components, including those components known as gangliosides, located in its plasma membrane. Gangliosides contain a particular type of acidic carbohydrate known as sialic acid. Further, many specific types of cells, including tumor cells, are characterized by the presence of a particular type of ganglioside located in their plasma membranes.
In recent years, a number of monoclonal antibodies have been established after immunization with human tumor cells or tissues. These monoclonal antibodies were selected by their positive reactivity to tumor cells and negative reactivity to normal cells or tissues. Many of the monoclonal antibodies selected by preferential reactivity to melanomas, neuroblastomas and adenocarcinomas have been identified as being directed to gangliosides. Some of these anti-ganglioside antibodies with specific isotopes (particularly IgG.sub.3 and IgG.sub.2a) and which show strong reactivity to gangliosides, have been found to suppress tumor growth in vivo. For example, melanomas of some patients have been found to regress following a large dose administration of a specific anti-GD.sub.3 ganglioside antibody (Houghton, A. N. et al, Proc. Natl. Acad. Sci. USA, 82:1242-1246 (1985)). Further, recently it has been demonstrated that GM.sub.2 absorbed on BCG bacteria showed a detectable immune response. Thus, it has been asserted that GM.sub.2 could be a useful vaccine for human melanomas (Livingston, P.O. et al, Proc. Natl. Acad. Sci. USA, 84:2911-2915 (1987)). Hence, gangliosides are important antigens and immunogens of tumor tissues and cells (Hakomori, S., Annu. Rev. Immunol., 2:103-126 (1984); Hakomori, S., In Handbook of Lipid Research, Volume 3, Sphingolipid Biochemistry, Kanfer, J. N. et al Eds., Plenum, N.Y., pages 1-165 (1983); and Hakomori, S., Sci. Amer., 254:44-53 (1986)).
However, the use of tumor cells (including cell membranes), tumor tissues, or isolated gangliosides absorbed on bacteria as immunogens, is extremely laborious and requires extensive selection studies. In addition, although gangliosides are important cell type-specific markers, they are poor immunogens in eliciting humoral or cellular immune responses. As a result, repeated immunization with tumor cells (including cell membranes), tumor tissues or isolated gangliosides absorbed on bacteria or other carriers is disadvantageously necessary.
A small portion of gangliosides are present in tumor cells and tissues in the form of a lactone thereof. For example, less than 0.1% of the particular ganglioside, designated GM.sub.3 (see FIG. 1A), present in melanoma cells has been identified as a lactone thereof (see FIG. 1B). Ganglioside lactones are defined as the inner ester between the carboxyl group of the sialic acid and the primary or secondary hydroxyl group of the sugar residues within the same molecule. One example of a GM.sub.3 lactone, wherein the carboxyl group of sialic acid is esterified with the C-2 secondary hydroxyl group of the penultimate galactose is shown in FIG. 1B (Yu, R. K. et al, J. Biochem. Tokyo, 98:1307 (1985)). This structure is sterically stable and relatively stable at acidic to neutral pH, although unstable at alkaline pH. While galactoside lactones have been detected and believed to be naturally occurring plasma membrane components, their quantity is extremely low and thus their natural occurrence has been disputed (Nores, G. A. et al, J. Immunol., 139:3171-3176 (1987) and Riboni, L., J. Biol. Chem., 261:8514-8519 (1986)).
Despite the question about their natural occurrence, it has been demonstrated in the present invention that ganglioside lactones are strong immunogens, which can cause a much greater immune response than native gangliosides. Further, it has been found in the present invention that the antibodies produced using ganglioside lactones as immunogens are of the IgG.sub.3 isotype, which is extremely useful, compared to antibodies of the IgM isotype produced using native gangliosides, (i) in detecting tumors containing gangliosides, (ii) in suppressing growth of tumors containing gangliosides in vitro and in vivo and (iii) in inducing antibody-dependent cytotoxicity in vivo. In addition, it has been found in the present invention that ganglioside lactones themselves are effective for suppressing growth of tumors containing gangliosides in vivo, whereas such suppression is not achieved using native gangliosides.